1. Field of the Invention
The present invention generally relates to charge and discharge protection circuits for a secondary battery and, more particularly, to a charge and discharge protection circuit for protecting a secondary battery from an overcharge state, an overdischarge state and an over-current state by detecting respective states.
2. Description of the Prior Art
FIG. 1 is a block diagram illustrating a charge and discharge protection circuit according to the related art.
For example, Japanese Laid-Open Patent Application No. 7-131938 discloses such a charge and discharge protection circuit and a battery pack using the same.
Referring to FIG. 1, the charge and discharge protection circuit according to the related art is constructed such that a voltage dividing circuit 1, an overcharge voltage detecting circuit 2, an overdischarge voltage detecting circuit 3 and a control circuit 4 are connected in parallel with a secondary battery (power supply). The control circuit 4 detects the state of the secondary battery by controlling the overcharge voltage detecting circuit 2 and the overdischarge voltage detecting circuit 3 so as to output a signal Vs for controlling the power supply to an external device or controlling charging of the secondary battery by an external power supply. The control circuit 4 further controls a switching element 5 provided in series with the voltage dividing circuit 1 so as to reduce a current flowing in the voltage dividing circuit 1.
According to the charge and discharge protection circuit of such a construction, the secondary battery is protected from an overcharge state, an overdischarge state and an over-current state by detecting respective states.
FIG. 2 is a sectional view of a transistor used to construct the charge and discharge protection circuit of FIG. 1.
The charge and discharge protection circuit according to the related art is constructed by integrating transistors having the Silicon on Insulator (SOI) construction as shown in FIG. 2.
A transistor having the SOI construction is constructed such that single-crystal silicon films 5C, 5D and 5E are formed on an insulating film 5B formed on an SOI substrate 5A. More specifically, the n-type source tare 5C and the n-type drain 5E are formed on respective sides of the channel area 5D. A gate electrode 5G is formed on the channel area 5D via a gate insulating film 5F. A protective film 5H covers the entirety of the SOI construction.
In such a charge and discharge protection circuit and a battery pack using the same, those transistors connected to terminals to which a charger is connected are required to have a high withstand voltage construction so that the secondary battery is properly protected in the event that an improper or broken charger is connected to the terminals. However, when the charge and discharge protection circuit is implemented by integrating the transistors all having the SOI construction as described above, the result is that the transistors not required to have a high withstand voltage (for example, transistors connected to the terminals connected to the battery) have a high withstand voltage. As a result, the chip area of the charge and discharge protection circuit is relatively large.